The present invention relates to a light emitting element and a method for manufacturing the same.
Materials used in light emitting elements such as a light emitting diode and a semiconductor laser and element structures thereof have been technically progressed for many years and as a result, an internal quantum efficiency in the interior of the element has been gradually closer to a theoretical limit. Thereto, in order to obtain an element with a higher luminance, an external quantum efficiency of the element is a very important factor. As for a method for improving the external quantum efficiency, there has been proposed a method in which a light transparent semiconductor substrate is bonded with the light emitting layer section so that light directed to the substrate side from the light emitting layer section can contribute to light emission. However in a case where a light transparent semiconductor substrate is bonded directly with a light emitting layer section, there arises a problem since a process therefor is generally apt to become complex and the light emitting layer section is subject to deterioration due to a necessary bonding treatment at a high temperature.
A light emitting element having a light emitting layer section made of AlGaInP mixed crystal adopts a double heterostructure in which a thin AlGaInP (or GaInP) active layer is sandwiched by an n type AlGaInP cladding layer and a p type AlGaInP cladding layer, each having a bandgap larger than the active layer, thereby enabling a high luminance element to be realized. Such an AlGaInP double heterostructure can be formed by growing layers made of AlGaInP mixed crystals epitaxially on a GaAs single crystal substrate using lattice-matching between AlGaInP mixed crystal and GaAs. In a case where the double heterostructure is used as a light emitting element, generally, a GaAs single crystal substrate is also in many cases used as an element substrate as it is. However, since AlGaInP layer mixed crystal or a light emitting layer section is larger in band gap than GaAs, there is a drawback that emitted light is absorbed in the GaAs substrate to cause a sufficient external quantum efficiency to be difficult to obtain. While in order to solve this problem, a method is proposed (for example, in JP A 95-66455) in which a reflective layer made of multiple semiconductor layers is inserted between a substrate and a light emitting element, a great improvement on external quantum efficiency cannot be expected in consideration of a principle since differences in refractive index between stacked semiconductor layers are used, and so only light with a limited angle range of incidence is reflected.
On the other hand, according to a very recently issued literature (Applied Physics Letters, 75 (1999) 3054), a proposal is made on a structure in which a metal layer made of mainly Au is inserted between an light emitting layer section having an AlGaInP double heterostructure and a silicon single crystal substrate as shown in FIG. 14. To be concrete, a light emitting element 100 shown in FIG. 14 has a structure in which an AuBe layer 103 and an Au layer 104 are formed as a metal layer 110 on an SiO2 layer 102 formed by oxidizing an n type silicon single crystal substrate 101; and thereon, further, a p type GaAs cap layer 105, a p type AlGaInP cladding layer 106, an AlGaInP active layer 107 and an n type AlGaInP cladding layer 108 forming a double heterostructure; and an electrode 109 made of an AuGeNi/Au layer are formed in the order. Light generated in the active layer 107 is reflected on the Au layer 104 as shown in FIG. 15.
With this structure adopted, since the metal layer 110 serves as a reflective mirror, a high reflectance not dependent on an incidence angle can be obtained thereby enabling an external quantum efficiency to be enhanced to a great extent. In this case, however, since the AlGaInP mixed crystal layer cannot be grown directly on the metal layer, the following method is adopted. The process goes this way: The silicon single crystal substrate 101 on which the metal layer 110 is formed by vapor deposition and a GaAs single crystal substrate on which a light emitting layer section including the AlGaInP double heterostructure 106, 107 and 108, and the GaAs cap layer 105 is epitaxially grown are separately prepared. Then, both substrates are bonded between the metal layer 110 and the cap layer 105 and thereafter, the GaAs single crystal substrate is removed, followed by forming the necessary electrodes to complete an element.
In the element disclosed in the above literature, a silicon single crystal substrate 101 on which the metal layer 110 is formed is covered with a thick insulating film 102 made of SiO; and as shown in FIG. 14 current supply to the cap layer 105 and the light emitting layer sections 106 to 108 is performed between the Au layer 104 and the electrode 109 using a portion exposed on the outer side of the Au layer 104 and a portion exposed on the outer side of the cap layer 105 and the light emitting layer sections 106 to 108 as electrodes, but not through the insulating film 102. Therefore, in this structure, a fault arises that a structure of a terminal lead of the element inevitably results in complexity, which in turn, leads to increase in man hours in manufacture and then to a higher cost of an element.
It is an object of the present invention to provide an light emitting element excellent in external quantum efficiency thereof and in addition, not only simple in structure of a terminal lead thereof but excellent in convenience; and a method for manufacturing the same.
In order to solve the above problems, a light emitting element of the present invention comprises a conductive substrate, a metal layer, a light emitting layer section and a first electrode, wherein the metal layer, the light emitting layer section and the first electrode are formed in the order on a first main surface side of a conductive substrate and a current can be supplied to the light emitting layer section through the first electrode and the conductive substrate.
According to the above structure, since the metal layer is inserted between the substrate and the light emitting layer section, a reflection on the metal layer can be used, thereby enabling not only a good external quantum efficiency to be realized, but electrodes or terminals to be formed on both sides of the light emitting element. That is, dissimilar to the light emitting element of the above literature (FIG. 14), no necessity arises for adopting a complex structure that the metal layer is exposed on the side of the light emitting layer section to form a terminal lead section. Therefore, a structure of a terminal lead of an element is greatly simplified, thereby enabling downsizing a chip of a light emitting element, but also realization thereof excellent in convenience.
A direction of current through a stacked layer body 9 constructed of a conductive substrate 2, a metal layer 3 and a light emitting layer section 4 can be any of a direction with which a first electrode side has a negative polarity as shown in FIG. 1A and a direction with which the first electrode side has a positive polarity as shown in FIG. 1B. In this case, the order of stacked layers in a heterojunction structure of the light emitting layer section 4 is reversed between the structures of FIGS. 1A and 1B.
The conductive substrate 2 can be made of a semiconductor such as silicon single crystal and also of a metal such as Al. In a case where the conductive substrate 2 is made of semiconductor, as shown in FIGS. 1A and 1B, a second electrode 6 is formed on a second main surface side of the conductive substrate 2 and a second terminal 12 is further formed on the second electrode 6. In this case, a current flows between the first electrode 5 and the second electrode 6. On the other hand, in a case where the conductive substrate 2 is made of a metal, since the second terminal 12 can be formed directly in the conductive substrate 2, the second electrode 6 can be omitted. Note that in a case where a semiconductor is adopted as a material of the conductive substrate 2, a structure is desirably adopted that the conductive substrate 2 and the metal layer 3 are put into direct contact with each other from the view points of current supply through the conductive substrate 2 without any trouble and enhancement of a bonding strength between the metal layer 3 and the conductive layer 2.
The first electrode 5 can be formed so as to cover only a part of the surface of the light emitting layer section 4. In this case, it is possible that of light beams 13 and 14 generated in the active layer of the light emitting layer section 4, at least a part of the light beam 14 directed to the metal layer 3 is reflected on the metal layer 3 and the reflected light 15 is leaked out through a region that is not covered with the first electrode 5 on the surface of the light emitting layer section, thereby, enabling better external quantum efficiency to be realized.
The above light emitting element 1 can be manufactured by superimposing the conductive substrate 2 and the light emitting layer section 4 on each other between bonding surfaces thereof with only the metal layer 3 interposed therebetween to apply a bonding treatment to the superimposed structure for manufacture.
To be concrete, the above light emitting element 1 can be manufactured according to a manufacturing method of the present invention. That is, a method for manufacturing a light emitting diode comprises the steps of; growing the light emitting layer section 4 epitaxially on a semiconductor single crystal substrate; bonding a first main surface of the conductive substrate 2 and a first main surface of the light emitting layer section 4 to each other with only the metal layer 3 interposed therebetween; and separating or removing the semiconductor single crystal substrate, wherein the steps are performed in the order. The bonding treatment can be, for example, a bonding treatment by heating.
In the above method, the conductive substrate 2 and the light emitting layer section 4 are bonded to each other with only the metal layer 3 interposed therebetween. Dissimilar to the above literature, by bonding the conductive substrate 2 and the light emitting layer section 4 to each other with only the metal layer 3 interposed therebetween, but without interposing an insulating film such as SiO2 or the like, not only can a bonding strength be strengthened but a good electrical conductive state between the conducive substrate 2 and the metal layer 3 can also secured. Especially, in a case where the conductive substrate 2 is made of silicon single crystal, compound semiconductor single crystal or mixed crystal, a higher boding strength can be assured of the substrate and part of the metal layer are bonded to form alloy.